Electromagnetic heating according to an efficiency of energy transfer

ABSTRACT

A method of delivering energy to an object in a cavity may include transmitting electromagnetic energy to the cavity at a plurality of frequencies. At each of the plurality of frequencies, the method may also include determining an efficiency of energy transfer into the object and adjusting power transmitted at each of the plurality of frequencies such that a multiplicative product of the efficiency and the power transmitted is substantially constant across each of the plurality of frequencies.

RELATED APPLICATIONS

The present application is a US Continuation Application of PCT PatentApplication No. PCT/IL2007/000236, filed on Feb. 21, 2007, which claimspriority of U.S. Provisional Patent Application No. 60/775,231, filed onFeb. 21, 2006 and U.S. Provisional Patent Application No. 60/806,860,filed on Jul. 10, 2006. The contents of the above-referencedapplications are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention is concerned generally with heating of materialswith electromagnetic energy.

BACKGROUND OF THE INVENTION

The microwave oven is a ubiquitous feature in modern society. However,its limitations are well known. These include, for example unevenheating and slow absorption of heat. In fact, ordinary microwave ovens,when used for heating (e.g. defrosting), cause temperature differencesas high as 100° C. between different locations in the heated object,resulting in creation of hotspots, regions of thermal runaway. Foreexample, frozen foods that are thawed in a microwave oven may have oneor more part (e.g. the outside) that is warm or even partly cookedbefore or other parts (e.g. in the interior) are even defrosted. Alsoknown are hotspots that occur within a heated cup of liquid that mayresult in personal injury to a user. One common method that attempts toreduce hot-spots is to rotate the article being heated. This method doesnot provide uniform heating as would be desired.

One method of providing uniform heating is to allow the heat depositedin a hot spot to diffuse to surrounding regions and heat them byconduction. Such methods may include an intermittent heating procedurein which the heating is periodically stopped to allow diffusion of heat.While this method may be used in conjunction with the methods of thepresent invention, by itself the stop and go method of heating is eitherextremely slow (due to the low heat conductivity of most foods, whichrequire long stop periods to make the method effective) or arerelatively ineffective. Another method is to heat at a very low power.This can be used, for example, with large frozen bodies. If the heatingis slow enough, then the excess heat at hot spots diffuses before thetemperature rise at the hot spot becomes objectionable. However, thismethod requires up to 10 or 20 times as much time for heating to befully effective. Due to convection from the object, it is not a seriousoption for cooking or heating much above room temperature.

A number of papers have been published in which a theoretical analysisof the problem of microwave warming of a cryogenic sample has beencarried out. Because of the difficulties of such analysis, such analysishas only been carried out on regular shapes, such as spherical andellipsoidal shapes. Experimental attempts have apparently been made onkidney sized specimens, but results of these experiments do not indicatethat a viable solution for defrosting kidneys is available.

Moreover, there does not appear to be a solution for defrosting otherorgans, or for defrosting warming or cooking foods, of more arbitraryshapes.

Prior art publications include:

-   S. Evans, Electromagnetic Rewarming: The effect of CPA concentration    and radio source frequency on uniformity and efficiency of heating,    Cryobiology 40 (2000) 126-138-   S. Evans, et al., Design of a UHF applicator for rewarming of    cryopreserved biomaterials, IEEE Trans. Biomed. Eng. 39 (1992)    217-225-   M. P. Robinson, et al., Rapid electromagnetic warming of cells and    tissues, IEEE Trans. Biomed. Eng. 46 (1999) 1413-1425-   M. P. Robinson, et al., Electromagnetic re-warming of cryopreserved    tissues: effect of choice of cryoprotectant and sample shape on    uniformity of heating, Phys. Med. Biol. 47 (2002) 2311-2325.-   M. C. Wusteman, Martin et al., Vitrification of large tissues with    dielectric warming: biological problems and some approaches to their    solution, Cryobiology 48 (2004) 179-189.

A paper entitled “Control of Thermal Runaway and Uniformity of Heatingin the Electromagnetic Warming of a Cryopreserved Kidney Phantom” by J.D. J. Penfold, et al., in Cryobiology 30, 493-508 (1993) describes atheoretical analysis and experimental results. While some experimentswere apparently made with a kidney sized phantom, the main reportedresults are with a uniform spherical object.

As reported a cavity was fed with electromagnetic energy at 434 MHz fromthree orthogonal directions (x, y, z). The x and y feeds were providedfrom a same generator and a phase change was introduced so that thefield was circularly polarized. The frequency was varied in steps of 32kHz (apparently up to about 350 kHz maximum) to match the inputimpedance as it changed with increasing temperature.

All of the above articles are incorporated herein by reference.

SUMMARY OF THE INVENTION

The present inventors have realized that the measures taken by prior artinvestigators to provide uniform heating were inadequate and could not,by themselves, lead to a viable methodology for uniform heating (ordefrosting) of irregular shaped objects such as organs, foods or thelike. In particular it was discovered that the prior art suffered frommany problems. As used herein, the term irregular means objects thatdepart from spherical or ellipsoid shape by more than 5% RMS volume.

Conventional microwave ovens are configured to feed into the ovenchamber microwave energy that is essentially of a single frequency. Dueto device constraints the energy is fed at different frequencies in asmall range, normally between 2.4 and 2.5 MHz. The inventors realizedthat the constraints of using a substantially constant frequency, oreven tracking a single dissipation peak in a small frequency range,significantly limited the ability to achieve uniform heating. In fact,heating at a single frequency is found to be one of the main reasons ofhotspots. However, using different frequencies (using one or morefeeds), may improve the uniformity of heating.

While some proposed prior art heaters did utilize more than onemicrowave input, the frequency differences between the two inputs aresmall, less than 6 MHz.

The inventors also found that the structure of the cavity of aconventional microwave oven, and especially the mode structure of thecavity, inherently did not allow achievement of uniform heating. Ingeneral, the fields for a given mode in a cavity vary with position andthe heating varies with the strength of the fields.

In the art, attempts were made to set the parameters of the microwaveoven to match features of a heated object before heating begins.However, during heating features of a heated object (e.g. the tendencyto absorb energy of a given frequency) change. Hence the inventorsrealized that even if a heater was tuned to a heated object beforeoperation, after even a short period of operation the features of theobject will have changed and the tuning will no longer be significant.

Another problem is that at times, the absorption at a given location ofan object is higher as the temperature increases. This can give rise toa “thermal runaway” problem (even in conventional microwave oven),wherein a relatively hot place absorbs more than a colder one thuscontinuously increasing the temperature difference. When an effort ismade to tune the energy input of the device to the object's impedance,the efficiency of energy delivery into the object may be maximized, buthotspots are also generally increased.

The inventors also noted that known publications dealing withdissipation of energy deal with absorption of energy by the resonator(e.g. surface currents) and not necessarily the object. Furthermore, noattention was drawn to the distribution of dissipation of energy in theobject (with the exception of some discussion of penetration depth).

Furthermore, when feeding from multiple directions into a cavity,coupling between the feeds can be a major problem. While for sphericalsamples these effects are minimal, for even moderate variations fromthis shape, the coupling between inputs can be quite large. Suchcoupling caused a number of problems including uneven heating and lowpower efficiency.

Some exemplary embodiments of the invention deal with one or more ofthese problems.

As used herein the term “heating” means delivering electromagnetic (EM)energy into an object. At times, an object may be heated according tothe present invention without temperature increase (e.g. when it isconcomitantly cooled at a rate that is at least equal to the heatingrate or at a phase change where the transmitted energy is taken up forthe phase change). Heating includes: thawing, defrosting, heating,cooking, drying etc, utilizing electromagnetic energy.

An aspect of some embodiments of the invention deals with more uniformheating of a real life, i.e., non-uniform or irregular geometry object.As used herein the term “object” means any object, including acomposition of one or more objects. In an embodiment of the invention,the hottest part of a thawed organ is 6° C. or less, when the coldestpart reaches 0° C. This has been confirmed with a cow liver. Inexperiments with a cow liver, after thawing from −50° C., the range oftemperatures in the thawed liver ranged from 8° C. to 10° C. In general,it is desirable to thaw the object such that all parts are abovefreezing point, to avoid recrystallization. In another embodimentobjects are heated to other temperatures (e.g. serving or cookingtemperatures, or a subzero temperature being above the temperature ofthe object before heating), while preserving a post heating uniformityof temperature within 50° C. At times, the uniformity of temperature ina heated (or thawed) object is maintained during heating such that atall times the uniformity of temperature is within 50° C. or even within10° C. or 5° C.

An aspect of some embodiments of the invention is concerned withsweeping the frequency of the feed (or feeds) over a finite set offrequency sub-bands (i.e. feeding energy into the heater over manyfrequencies belonging to each sub-band). For example, the dissipation ofenergy is measured for a band of RF frequencies (e.g. the wholeoperation range of the heater), and based on the measured results, afinite set of frequency sub-bands is selected. The width of band overwhich the energy efficiency is measured may for example be up to 2 GHz.At times, the band may have a width between 0.5% (5/1000 [MHz]) and 25%(100/400 [MHz]) of the center frequency.

The measurement may be performed before heating an object, at one ormore times during heating the object, or in advance (with a sampleobject to define the sub-bands for additional essentially identicalobjects). In an embodiment of the invention, RF energy is fed to thecavity at a plurality of frequencies and power levels responsive to theenergy efficiency measurements. For example, the input may be frequencyswept. Other methods described below may also be used.

An aspect of some embodiments of the present invention is concerned withassuring the efficiency of the heating process. The heating efficiencyis defined as portion of the power generated by an RF energy source(amplifier or other) that is absorbed in a heated object. Higherefficiency of the heating process results in a higher efficiency of thewhole process.

In an embodiment of the invention, the power coupled to other feeds ateach frequency in certain band (S_(ij)) and the return loss at eachfrequency (S_(ii)) are taken into account in determining the heatingefficiency and in adjusting certain characteristics of the apparatus,for example, a decision what power at what frequencies to transmit andthe timing of transmitting those frequencies at matching powers.Optionally, the absorbed power (input power less coupled power) fed intothe system from one feed is adjusted to be the same as the absorbedpower fed into each of the other feeds.

In an embodiment of the invention, the width of the efficiency“spectrum” (related to the Q factor) is desirably increased. It isknown, from the general theory of RF, that bigger loss in the object (orload) matches lower Q factor. In addition, wide dissipation peak allowsfor sweeping the frequency about the peak of efficiency, a techniquethat is believed to further improve the uniformity of heating. Based onthe band width, coupling between antennas and surface currents may bereduced. If dissipation is measured (even in an empty chamber) thedissipation peaks caused by antennas and/or metal components, and/orsurface currents appear as narrow dissipation peaks. Thus, by avoidingtransmittal in such bands (e.g. width being below 0.25% or even below0.75%) the energy loss may be reduced. Such measurement may be carriedout before and/or during heating of an object or during manufacture of aheater to prevent transmission of such wavelengths. Furthermore,coupling between inputs can be measured during manufacture and bandswith high coupling avoided.

In some embodiments of the invention, the power input to the feeds ateach transmitted frequency is adjusted to take into account differencesin power absorbed by the object being heated, which may serve to providea uniform or more uniform power absorption. Applicants have found thatchanging the transmitted frequency in some chosen sub-bands and theinput power at each frequency, within a those chosen sub-bands,optionally about the absorption peaks, results in a change in theheating pattern within the heated object. Thus, by sweeping thefrequency in chosen sub-bands, while the powers are properly adjusted,various portions of the object are heated. Keeping the total energyabsorbed in different locations of an object uniform results in moreeven heating of the object.

An aspect of some embodiments of the invention is concerned with thedesign, construction and calibration of a cavity for RF heating. Thecavity may be designed in order to meet certain needs of the presentinvention.

In an embodiment of the invention, the RF heater comprises one, two ormore electromagnetic energy feeds that feed energy to the cavity.Optionally, the feeds are antennas, preferably, wideband and/ordirectional antennae. Optionally the feeds are polarized in differentdirections to reduce coupling. These characteristics may be used tolower the coupling and provide a higher degree of freedom in working theinvention. In an exemplary embodiment of the invention three feeds whichare placed parallel to orthogonal coordinates are used. Optionally twoor more than three, for example six feeds are used. Optionally, only two(or in some embodiments even one) feeds are provided, when a lesseruniformity is acceptable and utilizing other aspects of the inventionprovides sufficient uniformity.

In some embodiments, rather than using an antenna having a single mainwire, through which the incoming wave reaches all parts of the antennastructure (which can be an array of antennas) several antennas may beused. This group of antennas may be operated as an antenna array bydelivering energy to each of the six antennas at a different time, thusmatching the phase resulting from the geometrical design of the complexantenna. This allows summing the RF energy on the object versus summingit before the antenna. Among the benefits of such groups of antennas isthe potential reduction of production costs (cheaper amplifiers). Inaddition, a possibility to control the phases of each input dynamically(and independently) provides an additional degree of freedom incontrolling the RF (EM) modes.

Furthermore, it is noted that an antenna array would normally have abigger area than a single antenna. A possible advantage would bereducing the dependence of location of a heated object on the heatingprotocol. Possibly two or more of the antenna sources are coherent,making the antenna structures have a common behavior. Furthermore, anantenna array may have a higher directionality or bandwidth and may thusprovide advantages in working the invention. Furthermore, arrays canoften be made steerable, to provide variable directionality of theantenna and to allow better transfer of energy to the object beingheated.

In some embodiments of the invention, a wide band solid state amplifiermay be used as an RF energy source. Among the potential benefits is thewide band of frequencies that may be introduced by the solid stateamplifier.

In an embodiment of the invention, at least one field adjusting elementis placed in the cavity to improve one or more parameters of the heatingprocess (e.g., coupling). Optionally more than one field adjustingelement is used. Optionally, any of the boundaries of at least one ofthe field adjusting elements is electrically floating (not touching themetal walls of the cavity). Optionally any part of the boundaries of atleast one element is attached to one of the walls of the cavity. In anexemplary embodiment of the invention, at least one of the elements isnot fixed in place, so that it can be moved and/or rotated and/orfolded/unfolded to improve one or more parameters of the heatingprocess. In an exemplary embodiment, of the invention, at least one ofthe elements rotates about an axis. In an exemplary embodiment of theinvention, the at least one element slides along a wall of the cavity.

In an exemplary embodiment of the invention the field adjusting elementis a metal or other conductor. Alternatively, any material, such as adielectric, optionally loaded with metal, which is known to perturbelectromagnetic fields, can be used as a matching element. The size,structure, place and material of a field adjusting element may affectthe effectiveness of the field adjusting element. The effect of the sizeis dependent also on the location of the element. At one location theeffect of the element on the measured energy transfer and other heatingparameters and in another it is not. In general, when the element is inthe direction of the directivity of the antenna it has a relativelylarge effect.

Additionally, the relation of height to radius of a chamber, and thegeometric design (e.g. box shape vs. cylinder shape) are known affectthe dissipation pattern of the chamber and the modes within the chamber.In designing a device according to some embodiments of the presentinvention, a simulation or trial error measurement of dissipation may beused to select a chamber being better suited, e.g. having widerdissipation peaks (low Q factor) in the object, or more adaptable (i.e.enabling a more dramatic change of the dissipation pattern, usingsimilar field adjusting elements, for example as detailed below) for thedesired heating.

An aspect of some embodiments of the invention is concerned with feedsused for feeding a cavity. According to an embodiment of the invention,energy is fed into the cavity via a coaxial input and the centerconductor of the coaxial input is extended past the wall of the cavityto form a partial loop. In an exemplary embodiment of the invention, theend of the extension is not attached to the wall of the cavity.Optionally, the partial loop comprises an antenna that radiates towardthe position of the object being heated to improve power transfer to theobject.

According to another embodiment of the invention, the energy is fed intothe cavity via a helical antenna optionally fed via a coaxial input.Optionally, the helix period, its diameter and/or its orientation areadjustable, thereby changing the modes and dissipation within thechamber. In some embodiments of the invention, one or more of the inputsutilize a right hand rotating helix while others utilize a left handrotating helix. This may minimize coupling between the helices.Alternatively, all helices have the same orientation.

According to yet another embodiment of the invention, fractal antennasare used at one or more of the inputs.

According to some additional embodiments of the invention, differentantenna types are used at different input ports.

In accordance with some embodiments of the invention antennas aredesigned according to a wavelength correction factor that converts thefree space center wavelength of an antenna to the effective centerfrequency in the cavity. The inventors have found that this conversionis substantially independent of the shape or size of the object beingheated.

An aspect of some embodiments of the invention relates to a method ofcontrolling the input of electromagnetic energy to a cavity of a heater.

In an exemplary embodiment of the invention one or more characteristicsof the heater are adjusted during heating of an object, responsive tochanges in the object or during initial adjustment of the heater. In anexemplary embodiment at least one of the (i) position and/or orientationof at least one field adjusting element and/or (ii) at the power oftransmission in at least one frequency (or sub-band of frequencies)and/or (iii) characteristics of one antenna structure or more and/or(iv) the location of the heated object are adjusted to improve the netpower and/or efficiency and/or uniformity of energy transfer to theobject being heated. Optionally, two or more of input frequency,position and/or orientation of at least one field adjusting element areadjusted.

In an exemplary embodiment of the invention, the frequencies of theinputs are substantially different. While in the prior art cited above,the frequencies are allowed to differ by up to 6 MHz, in the exemplaryembodiment of the present invention, the frequencies may differ by 10,20, 50, 100 or even several hundreds of MHz. This allows for greaterflexibility in providing power evenly to the object. In prior art, byimmersing the object in an anti-freezing liquid, uniformity of theobject was achieved. This resulted in a system in which thecharacteristics of the liquid were dominant, the frequency changedlittle during heating, but the object itself was not well matched to themicrowave environment. Moreover, at times it is preferred not to subjectthe object to uniformity induction (e.g. exposure to a fluid that mightbe hazardous to biological material or consumption or damage the tasteor structure of food).

Optionally, the chamber environment is controlled using conventionalenvironmental control elements (such as introduction of humidity,cooling or warming), is provided to the outside of the object. Suchexternal cooling may allow avoiding overheating of the outside.Alternatively, some heating may be provided to the outside to start thedefrosting process. This may help prevent recrystallization, or in thecase of an egg being boiled, the heating would reduce the temperaturegradient (and therefore stress) across the egg shell, thus reducing thechances of cracking and bursting. Accordingly, in some embodiments ofthe invention, heat radiating, concentrating or reflecting elements areprovided on the outside or within the object being heated. Control ofthe humidity can provide moisture to the object being heated to avoiddrying out of the object. For some objects, such as meat, it can cause amoisture retaining layer to be formed on the object, to avoid drying outof the object.

In some embodiments of the invention, RF sensitive objects are placed onor near the object being heated. Such object may act as passive sources.Examples of such sources include a rod of metal, which acts as a dipoleradiator or a metal powder which may be used as a reflector or a pieceof foil which may shield a small portion of the object being heated.

In an aspect of some embodiments of the invention, the end of heating(e.g. the end of defrost or boiling) is automatically detected and theheating stopped. Alternatively, during heating, the characteristics ofthe heating process may be adjusted to take the dielectric propertiesinto account (e.g., more power is transmitted at the phase change toavoid spending a long time in this process). In an embodiment of theinvention, the phase change is sensed by a change in dielectricproperties of the object; for example, as they are represented byvarious measurements of return loss and coupling of the feeds or adesired operating frequency. Optionally, the object may be encased in abag which will comprise temperature sensors. Optionally, a thermocouple,IR sensor and/or optical sensor are used to determine end of defrost,cooking or other heating processes.

Optionally, during heating, current temperature of an object isdetermined, based on the amount of RF power needed for a certain kind ofan object and an exact measurement of the RF power absorbed in theobject, through the continuous knowledge of the efficiency of powertransfer and the power into the feeds of the cavity.

An aspect of some embodiments of the invention relates to providing amicrowavable package, wrapper, tag, attachment or other indicatorincluding heating instructions which indicate a desired driving profilefor RF signals, rather than mere power as a function of time. In anexemplary embodiment of the invention, the indicated profile comprisesan index to a table or a simulation which generates RF driving profilesincluding multiple frequencies. Optionally, at least 3, at least 5 ormore distinct frequencies and/or one or more ranges of frequencies areindicated. Optionally, the driving profiles include one or more ofnumber of inputs to use, phase of the input(s), temporal schedule and/orpackage relative information, such as package thermal and RF behavior.

In an exemplary embodiment of the invention, resonant circuits areembedded in the object and/or on its surface (as for example in a bag inwhich the object is packaged). Such sensors may be identified byperforming a frequency scan and looking for a change in input impedanceat the resonant frequency. Such circuits can be used to identify theobject.

If the bag is provided with temperature sensitive elements, then theycan also be used to determine temperature (and detect the end and/orprogress of the heating process). Optionally, the frequency of thesecircuits is far from frequencies generally used for heating.Alternatively, the heater is configured so as not to transmit power inthe frequency that interacts with the specific resonance structure(while potentially transmitting higher and lower frequencies).

There is thus provided, in accordance with an embodiment of theinvention, an electromagnetic heater for heating an irregularly shapedobject, comprising:

a cavity within which an object is to be placed;

at least one feed which feeds UHF or microwave energy into the cavity;and

a controller that controls one or more characteristics of the cavity orenergy to assure that the UHF or microwave energy is deposited uniformlyin the object within ±30%, 20% or 10% over at least 80% or 90% of thevolume of the object.

Optionally, the at least one feed comprises a plurality of feeds.

In an embodiment of the invention, the one or more controlledcharacteristics include a frequency of the energy inputted at one ormore feeds. Alternatively or additionally, the one or more controlledcharacteristics include a position or orientation of a field adjustingelement inside the cavity. Optionally, the characteristics arecontrolled to provide a desired net efficiency of power into the cavity.

There is further provided, in accordance with an embodiment of theinvention, a method of heating an irregularly shaped object, the methodcomprising:

placing the object in a cavity of a heater;

feeding UHF or microwave energy into the heater;

controlling one or more of the characteristics of the cavity or energyto assure that the UHF or microwave energy is deposited uniformly in theobject within ±30%, 20% or 10% over at least 80% or 90% of the volume ofthe object.

In an embodiment of the invention, the one or more controlledcharacteristics include a frequency of the energy inputted at one ormore feeds. Alternatively or additionally, the one or more controlledcharacteristics include a position or orientation of a field adjustingelement inside the cavity. Optionally, the characteristics arecontrolled to provide a desired net efficiency of power into the cavity.Optionally, controlling the frequency comprises feeding energy at aplurality of frequencies covering a band of at least 0.5%.

On an embodiment of the invention, is frozen prior at the commencementof heating. Optionally, the object is heated until thawed. Optionally,the temperature differential in the object when thawing by said heatingis complete throughout the object is less than 50° C., 20° C., 10° C.,5° C. or 2° C. In an embodiment of the invention, the frozen object isan animal or human organ.

There is further provided, in accordance with an embodiment of theinvention, a method of heating an object in a cavity having at least oneRF port, the method comprising:

feeding energy into at least one port; and

varying the frequency of the energy during heating of the object so thatit varies over a band greater than 0.5%, 2%, 5%, 10% or 20%.

In an embodiment of the invention, the frequency is swept across theband.

Optionally, the band is at least 20 MHz or 100 MHz wide.

There is further provided, in accordance with an embodiment of theinvention, electromagnetic heating apparatus, comprising:

a cavity;

at least one UHF or microwave energy feed; and

at least one adjustable field adjusting element situated within thecavity.

Optionally, the at least one field adjusting element is a metal element.

Optionally, the at least one adjustable field adjusting element isrotatable to produce a desired power coupling. Alternatively oradditionally, the at least one field adjusting element is able to slideto produce a desired power coupling. Optionally, the at least oneadjustable field adjusting element comprises a plurality ofindependently adjustable elements.

There is further provided, in accordance with an embodiment of theinvention, a method for electromagnetic heating, comprising:

placing an object to be heated into a cavity;

feeding UHF or microwave energy into the cavity; and

adjusting a characteristic of the cavity to achieve a desired uniformityof heating.

Optionally, the cavity comprises at least one adjustable field adjustingelement within the cavity; and

wherein adjusting the cavity comprises adjusting the at least one fieldadjusting element.

Optionally, the at least one adjustable field adjustable elementcomprises a plurality of said elements.

Optionally, adjusting is performed at least once as heating proceeds.

There is further provided, in accordance with an embodiment of theinvention, apparatus for electromagnetic heating comprising:

a cavity;

a plurality of feeds (optionally 2, 3 or 6) which feed UHF or microwaveenergy into the cavity;

a controller that determines the efficiency of net power transfer intothe cavity and adjusts the frequency of the plurality of inputs suchthat the efficiency of net power transfer into the cavity is controlled.

Optionally, the controller adjusts the frequency during the periodbetween commencement and ending of heating.

Optionally, the apparatus comprises at least one adjustable fieldadjusting element situated in the cavity. Optionally, the controlleradjusts the field adjusting elements to enhance the efficiency of netpower transfer.

Optionally, the controller adjusts the frequency as heating proceeds.

Optionally, the controller is configured to feed at least two of thefrequencies at different power.

Optionally the controller sweeps the frequency as heating proceeds.

There is further provided, in accordance with an embodiment of theinvention, a method of electromagnetic heating comprising:

placing an object to be heated into a cavity;

feeding UHF or microwave energy into the cavity via a plurality offeeds;

determining the efficiency of net transfer of energy into the cavity foreach of the feeds as a function of frequency over a range offrequencies; and

adjusting the frequencies of the energy fed, responsive to thedetermined efficiency function.

In an embodiment of the invention, the method includes adjusting thefrequency as heating proceeds.

Optionally, the method includes sweeping the frequency over the band.

Optionally, the method includes adjusting the power at each feedresponsive to the efficiency function as the frequency is adjusted.

In an embodiment of the invention, the overall efficiency of energytransfer into the object to be heated as compared to the energy fed intothe feeds is greater than 40% or 50%.

There is further provided, in accordance with an embodiment of theinvention, apparatus for electromagnetic heating comprising:

a cavity;

at least one feeds which feed UHF or microwave energy into the cavity;

a controller that determines a change in a desired frequency of energyas heating proceeds and changes the frequency by at least one MHz, 10MHz or 25 MHz.

In an embodiment of the invention, the desired frequency change isdetermined from a measurement of the net efficiency of energy transferto the cavity over a band of frequencies.

There is further provided, in accordance with an embodiment of theinvention, a method of electromagnetic heating comprising:

placing an object to be heated in a cavity; and

changing a frequency of UHF or microwave energy fed into the cavity forheating the object by at least 1 MHz, 10 MHz, 25 MHz or 25 MHz duringthe course of the heating.

In an embodiment of the invention, the desired frequency change isdetermined from a measurement of the net efficiency of energy transferto the cavity over a band of frequencies.

In an embodiment of the invention, the frequency is swept over at leastone sub-band of frequency of at least 5 MHz.

In an embodiment of the invention, the power is adjusted for eachfrequency responsive to the measurement of the net efficiency.

There is further provided, in accordance with a method ofelectromagnetic heating comprising:

placing an object to be heated into a cavity; and

feeding UHF or microwave energy into the cavity via a plurality offeeds;

wherein the frequencies of the energy fed to two of the feeds differs byat least 8 MHz, or 20 MHz.

In an embodiment of the invention, the net energy fed into the objectfrom each of the plurality of feeds is equal to within 25%.

There is further provided, in accordance with an embodiment of theinvention, a method of electromagnetic heating, comprising:

subjecting an object that is to be heated to UHF or microwave energy inan amount capable of heating the object;

determining a characteristic of the heating process that is responsiveto a change in state of the object; and

adjusting the heating when a desired state is achieved.

There is further provided, in accordance with an embodiment of theinvention, a method of electromagnetic heating, comprising:

subjecting an object that is to be heated to UHF or microwave energy inan amount capable of heating the object;

determining an amount of energy that is absorbed by the object; and

adjusting the heating when a desired amount of energy is absorbed.

There is further provided, in accordance with an embodiment of theinvention, apparatus for electromagnetic heating comprising:

a cavity;

at least one feed for UHF or microwave energy; and

a source of static or low frequency electric or magnetic field arrangedto subject an object in the cavity to an electric or magnetic field,effective to affect the heating of an object in the cavity.

There is further provided, in accordance with an embodiment of theinvention, a method of electromagnetic heating comprising:

subjecting an object to be heated to UHF or microwave energy in anamount suitable for heating the object; and

subjecting the object during heating to a static or low frequencyelectric or magnetic field effective to increase the uniformity orefficiency of heating.

There is further provided, in accordance with an embodiment of theinvention, apparatus for electromagnetic heating comprising:

a cavity;

at least one feed into the cavity that includes an antenna including aradiating element chosen from the group consisting of a patch antenna, afractal antenna, a helix antenna, a log-periodic antenna, a spiralantenna and a wire formed into a partial loop that does not touch a wallof the cavity.

In an embodiment of the invention, the radiating element comprises anarray of radiating elements.

In an embodiment of the invention, the at least one feed comprises aplurality of feeds and wherein the radiating elements of at least twofeeds is different.

There is further provided, in accordance with an embodiment of theinvention, a method of producing selective heating on a portion of anirradiated object comprising:

providing an object to be heated;

providing an energy concentrating element on, in or near the object;

placing the object and the energy concentrating element in a resonantcavity; and

irradiating the object and the element to cause a concentration ofenergy at selected places in the object.

Optionally, the energy concentrating element is irradiated at afrequency at which it is resonant.

Optionally, the object and the element are placed in the cavityseparately.

There is further provided, in accordance with an embodiment of theinvention, an RF heater comprising:

a resonant cavity;

at least one source of microwave or UHF energy;

at least one feed for feeding energy generated by the at least onesource into the cavity;

a power supply for the at least one source; and

a housing for the RF heater,

wherein the RF heater weighs 15 Kg, 10 Kg, 7 Kg or less.

In an embodiment of the invention, the resonant cavity has a volume ofat least 20, 30 or 40 liters.

There is further provided, in accordance with an embodiment of theinvention, a method of determining the temperature of a portion of anobject being heated in an RF heater, comprising:

placing the object in resonant cavity of the heater;

providing a temperature sensitive sensor having a resonant frequencythat varies with temperature;

irradiating the object with UHF or Microwave power via a feed; and

determining the temperature based on energy reflected from the feed.

In an embodiment of the method comprises:

placing a non-temperature sensitive resonant element adjacent to thetemperature sensitive element,

wherein determining comprises determining based on a frequencydifference between resonances of the temperature sensitive sensor andthe non-temperature sensitive resonant object as indicated by saidreflected energy.

In embodiment of the invention, the method comprises: controllingcharacteristics of the irradiation of the energy responsive to thedetermined temperature.

There is further provided, in accordance with an embodiment of theinvention, a method for RF heating of an object in a cavity, comprising:

irradiating the object with UHF or Microwave energy;

adjusting the humidity of or cooling the air in the cavity.

In an embodiment of the invention adjusting the humidity of or coolingthe air in the cavity comprises adjusting the humidity of the air in thecavity. Additionally, the temperature may be adjusted. Alternatively oradditionally to adjusting the humidity adjusting the humidity of orcooling the air in the cavity comprises cooling the air in the cavity.

There is further provided, in accordance with an embodiment of theinvention, an RF heater comprising:

a resonant cavity;

at least one RF source having a power output of at least 50 watts andbeing sweepable over a frequency range of greater than 0.5% with anefficiency of greater than 40%

at least one feed for feeding energy generated by the at least onesource into the cavity;

a power supply for the at least one source; and

a housing for the RF heater.

Optionally, the RF source comprises:

a signal generator that produces selective frequencies within the band;and

an RF amplifier.

Optionally, the at least one RF source comprises a plurality of sources.

Optionally, the at least one feed comprises a plurality of feeds.

Optionally, the at least one RF source comprises one or both of a UHFsource or a Microwave source.

Optionally, the source is sweepable over a frequency range greater than2%, 5%, 10%, 20% or 25%.

Optionally, the power output available for each feed is at least 200Watts or 400 Watts.

There is further provided, in accordance with an embodiment of theinvention, an RF heater comprising:

a resonant cavity;

at least one RF source having a power output of at least 50 watts andbeing sweepable over a frequency range of greater than 200 MHz with anefficiency of greater than 40%

at least one feed for feeding energy generated by the at least onesource into the cavity;

a power supply for the at least one source; and

a housing for the RF heater.

Optionally, RF source comprises:

a signal generator that produces selective frequencies within the band;and

an RF amplifier.

Optionally, the at least one RF source comprises a plurality of sources.

Optionally, the at least one feed comprises a plurality of feeds.

Optionally, the at least one RF source comprises one or both of a UHFsource or a Microwave source.

Optionally, the power output available for each feed is at least 200Watts or 400 Watts.

There is further provided, in accordance with an embodiment of theinvention, a package suitable for use in an RF heating oven, comprisingat least one indicator having a machine-readable indication of heatinginstructions thereon, which indication indicates uniform or controlledheating instructions.

In an embodiment of the invention, the machine readable indication isreadable by a scanning RF field in an RF cavity.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary non-limiting embodiments of the invention are described belowwith reference to the attached figures. The drawings are illustrativeand generally not to an exact scale. The same or similar elements ondifferent figures are referenced using the same reference numbers.

FIGS. 1A, 1B and 1C are respective schematic top and side section viewsof a cavity 10, in accordance with an exemplary embodiment of theinvention;

FIGS. 2A and 2B show two exemplary matching elements, in accordance withan embodiment of the invention;

FIG. 3 is a schematic isometric drawing of the interior of the cavity ofFIG. 1;

FIG. 4A is a schematic drawing of an antenna useful for coupling energyinto the cavity, in accordance with an embodiment of the invention;

FIG. 4B is a schematic drawing of a helical antenna useful for couplingenergy into the cavity, in accordance with an embodiment of theinvention;

FIG. 4C shows a graph of correlation of free space matched frequenciesand cavity matched frequencies of a helical antenna feed;

FIGS. 4D-4H are schematic drawings of various fractal antennas usefulfor coupling energy into the cavity, in accordance with an embodiment ofthe invention;

FIGS. 5A-5C are schematic block diagrams of electromagnetic heatingsystems, in accordance with an embodiment of the invention;

FIG. 6 is a simplified flow chart of the operation of the system, inaccordance with an embodiment of the invention;

FIG. 7 is a flow chart of a process of adjusting elements and frequencyin the heating systems illustrated in FIG. 5, in accordance with anembodiment of the invention;

FIG. 8 illustrates alternative RF circuitry, in accordance with anembodiment of the invention;

FIG. 9 is a graph of frequency vs. time for a typical thawing process,illustrating an automatic turn-off capability in accordance with anembodiment of the invention;

FIG. 10 shows the layout of a low frequency bias structure, inaccordance with an embodiment of the invention;

FIG. 11A is a simplified flow chart of a method of determining sweptpower characteristics, in accordance with an embodiment of theinvention;

FIGS. 11B and 11C illustrate how a swept efficiency spectrum isdetermined, in accordance with an embodiment of the invention;

FIG. 11D shows a pulse shape, power spectrum for a pulse operative toprovide the spectrums shown in FIGS. 11B and 11C, in accordance with anembodiment of the invention;

FIG. 12A shows an RF heater with an auxiliary heating coil, inaccordance with an embodiment of the invention;

FIGS. 12B and 12C schematically illustrate a scheme for transferringwaste heat from an amplifier to the heater of FIG. 12A; and

FIG. 12D shows an external view of a low weight, high efficiency RFheater, in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The present application describes a number of advances in the field ofRF heating (e.g. microwave or UHF) heating. While, for convenience theseadvances are described together in the context of various apparatus andmethods, each of the advances is generally independent and can bepracticed with prior art apparatus or method (as applicable) or with anon-optimal version of the other advances of the present invention.Thus, for example, parts of the method of adjusting the input power canbe used with the prior art apparatus of Penfold, et al., referencedabove. Conversely, the inventive apparatus of the present invention (orparts thereof) can be used with the method of Penfold et al. It isexpected that these combinations will not be ideal, but they areexpected to give improved results over the prior art apparatus andmethods.

Furthermore, advances described in the context of one embodiment of theinvention can be utilized in other embodiments and should be consideredas being incorporated as optional features in the descriptions of otherembodiments, to the extent possible. The embodiments are presented insomewhat simplified form to emphasize certain inventive elements.Furthermore, it is noted that many features that are common to most orall embodiments of the invention are described in the Summary of theInvention and should also be considered as being part of the detaileddescription of the various embodiments.

The following are believed to novel features or variations present insome or all the embodiments described. It is understood that not all ofthese features may be present in any particular embodiment and that notall features are described for every embodiment for which they areapplicable.

1) An apparatus and method that allow for RF heating an irregular objectsuch that the temperature of the object is uniform within 50° C.(optionally, to within 10, 6, 4 or 2° C.) when heating is completed.Exemplary embodiments provide this uniformity mainly by directly RFheating the object such that over 50% of the heating is by direct RFheating and not by conduction from other portions of the device. In someembodiments of the invention, such direct RF heating can reach 70, 80,or 90 or more percent.

2) An apparatus that includes field adjusting elements inside the cavityand method of designing and using same.

3) A heating apparatus with one or more coupling antenna for couplingenergy into the cavity; a method of designing said antenna; and methodof feeding energy to the heater including a method of tuning theradiated pattern of the antenna. This includes, utilizing an antennaarray (with one or more feeds, having controlled phases), loop antenna,wide band antenna, fractal antenna, directional antenna, helix antenna,operating the antennas separately or coherently, designing the antennato obtain a desired radiated pattern etc.

4) An apparatus and method to gain knowledge of a heating processbefore, and potentially also several times during, heating (e.g. severaltimes a second) using a measurement of the efficiency of absorption ofenergy in the object being heated as function of frequency

5) An apparatus and method that is adapted to control one or morecharacteristics of the heating process, for example the amount of powerabsorbed in the heated object, based on the measurement of energyabsorption efficiency (e.g. by transmitting power to compensate for thevariations of energy absorption). This may be done by adjusting, forexample, input power at each transmitted frequency and/or choosingfrequencies to be transmitted and/or moving the field adjustingelement's and/or moving the heated object and/or changing the antennascharacteristics. This may be done before operation, and preferably alsoone or more times during operation (e.g. several times a second), basedon measurements of energy absorption during heating or during a shorthiatus in the heating.

6) An apparatus and method for applying a DC or low frequency electric(e.g. below 300 MHz, or below some other value substantially lower thatthe heating frequencies used) or magnetic field to the object during RFheating. Such application is believed to change the dielectricproperties of the object being heated and this provides yet anothermethod of adjusting the power provided to the object being heated.

7) An apparatus and method in which during operation the transmittedfrequencies and/or power from one or more feeds are varied in acontrolled manner to get a desired heating pattern (e.g. by more than 1,2 or 5 MHz). This variation may occur several times during operation(e.g. several times a second). In an embodiment of the invention, thedesired pattern is a uniform heating pattern.

8) Apparatus and method of controlling heating based on reading ofdielectric characteristics of the heated object. Reading may be obtainedone or more times during heating (e.g. several times a second). Forexample end of thawing or boiling process, when a phase change issensed. This can implement a cessation of heating.

9) An electromagnetic heater including multiple inputs in which thefrequencies of the inputs are different by more than 5, 10 or 25 MHz.

10) An electromagnetic heater including multiple inputs in which thefrequencies of at least one of the inputs changes dynamically duringheating such that the frequencies at the inputs vary by 5 MHz or more.

11) An apparatus that utilizes a wideband and high efficiency (above40%) solid state microwave amplifier to feed energy into the cavity andoptionally utilize waste heat generated by the generator to heat the airin the cavity.

12) An apparatus that utilizes wasted heat generated by the RF energygenerator to heat a medium, for example air in the cavity, or water, asin a water heater.

13) A method of causing a resonance structure and/or designed pattern,inside a resonator to radiate by (selectively or generally) irradiatingsaid resonance structure and/or designed pattern thus using it as aradiation source (i.e. creating a passive source) and an apparatuscomprising same.

14) Apparatus and method of using RF reflecting object, such as metals,for concentration of energy in close environment of these objects,inside a resonator, for example within the heated object or in the closeenvironment of the heated object.

15) Apparatus and method of high-efficiency (at least 50%, at timesabove 70% or even 80%) RF heater. The efficiency is defined as powerabsorbed in the object versus power at the output of the power source.This opens the possibility of a heater that operates from a solar energysource.

16) An RF heater weighing less than 15 Kg, or even less than 10 Kg. Inaccordance with some embodiments of the invention a the use of a highefficiency solid state amplifier rather than a microwave tube allows forusing a low weight DC power source instead of the heavy dutytransformer. This heat saving is additional to the replacement of aheavy magnetron with a light solid state amplifier. Furthermore, thehigh efficiency eliminates the need for a heat sink, e.g. by using theresonator as a heat sink. In some embodiments of the invention, therequirement for a heat sink is obviated or partly reduced by feeding thewaste heat from the amplifier back into the microwave cavity.

17) Apparatus and method of temperature information of a heated objectusing a TTT (a temperature sensitive, preferably passive Temperaturetransmitting tag the resonance of which changes due to temperaturechanges or which transmits the temperature information using a modulatedresponse). This may be done if the TTT frequency is remote from thetransmittal range of the device, or if the TTT's frequency is within thedevice's band width, and avoiding the specific TTT frequencies duringheating. In some embodiments of the invention a tag having two resonantelements, one that is temperature sensitive and one that is not can beused since measurement of frequency difference is more accurate thanmeasurement of absolute frequency.

18) An apparatus and method for RF heating including means for chamberenvironment control (e.g. introduction and/or removal of humidity,cooling and/or warming etc.). For example, in the case of an egg beingboiled, heating would reduce the temperature gradient (and thereforestress) across the egg shell, thus reducing the chances of cracking andbursting. Optionally, the air temperature in the chamber may be variedwith time, depending on the present temperature of the object andobjectives such as causing condensation that closes the object beingheated (such as meat).

19) An apparatus in which the power absorbed by the object being heatedcan be calculated based on knowledge of power input and efficiency ofpower transfer to the object being heated. This allows for thecalculation of a current temperature and/or a turn off-time based onactual heating rather than some estimated heating time as presently usedwith microwave cookers.

FIGS. 1A, 1B and 1C show respective top and side section views of acavity 10, in accordance with an exemplary embodiment of the invention.

Cavity 10, as shown is a cylindrical cavity made of a conductor, forexample a metal such as aluminum, and is resonant in the UHF ormicrowave range of frequencies, optionally between 300 MHz and 3 GHz,more preferably between 400 MHz and 1 GHZ. In some embodiments of theinvention, the cavity is a spherical, rectangular or elliptical cavity.However, it should be understood that the general methodology of theinvention is not limited to any particular resonator cavity shape.

On one end 12 of the cylinder and on two sides of the cylindricalportion 14 feed antennas 16, 18 and 20 are positioned to feed energy ata frequency which is optionally chosen using the methods describedbelow. Various types exemplary but not limiting antennas useful incarrying out the invention are shown in FIGS. 4A-4C.

In an exemplary embodiment of the invention, one or more matchingelements 22, 24 are placed inside the cavity, optionally near the feedantennas. Two types of field adjusting elements are shown; however,other shapes and materials can be used. First field adjusting element22, shown more clearly in FIG. 2A is situated on end 12 of cavity 10. Inthis embodiment the element is rotatable about an axis 28 attached tothe end, in a direction 30. Optionally, it is insulated from the end byan insulating sheet 32 which couples element 22 capacitively to end 12.Alternatively it is conductively attached.

It is believed that element 22 (as well as the other field adjustingelement) has a dual effect, when properly adjusted. On the one hand itchanges the modes of the cavity in a way that selectively directs theenergy from the feeds into the object to be heated. A second and relatedeffect is to simultaneously match at least one of the feeds and reducecoupling to the other feeds.

Field Adjusting element 24, shown more clearly in FIG. 2B is situatedbetween feed 18 and end 12. One end of the element optionally iselectrically attached to cylindrical portion 14 of the cavity. The otherend of element 24 is spaced and insulted from end 12 by insulatingmaterial 36. It is free to slide along end 12 and cylindrical portion asshown by arrows 33 and 34. This sliding changes the spectral variationof the energy absorption efficiency.

FIG. 3 is a perspective drawing of the interior of the cavity to moreclearly show the position and orientation of the feed and elements.

FIGS. 4A-4H show three different types of antennas that are useful incarrying out the invention. These antennas are either novel per se, orif known have never been used for feeds in a microwave oven or heater,especially in a cavity type heater. In general, in most microwave cavitytype heaters, the feeds used are not directional to any great extent andnot wideband, as defined in free air. The object of the feeds is toexcite the modes of the cavity. Since the cavities of the prior art areexcited at a single frequency or a narrow band of frequencies, theantennas were designed specifically to excite these modes. In addition,prior art microwave cavities use waveguides or loop antennas which arenot designed to lower the coupling of energy from one feed to another(they generally have only a single feed). The present inventors havediscovered that the use of directional antennae and/or wideband antennaeallows for better coupling to the heated object and lower coupling toother feeds.

In some embodiments the antennas are supplied as arrays. There are someadvantages in using an antennas array. The band may be larger and thereis a lower dependence of the heated object location on the results. Thedirectivity may be controlled, even adjusted during heating. It ispossible to control the phase of every single antenna of the array,controlling the RF mode. It is possible to alter the antenna structure,for example, using the helix antenna, the radius and the height of theantenna may be changed in order to tune the impedance and change the RFmode.

FIG. 4A shows an antenna useful for coupling energy from feeds 16, 18and 20 into cavity 10, in accordance with an embodiment of theinvention. As shown, feed 16 includes a coaxial feed 37 with its centerconductor 38 bent and extending into the cavity. The center conductor isbent but does not touch the walls of the cavity. Optionally, the end ofthe wire is formed with a conductive element 40 to increase the antennabandwidth. The present inventors have found that antennas of the typeshown are able to couple energy better to an irregular object in thecavity. It is believed that such antennas transmit directionally and ifthe bend is aimed toward the object being heated, then coupling to theobject (as opposed to coupling to the cavity) will be improved.

FIG. 4B shows a helix antenna 41 useful for coupling energy from feeds16, 18 and 29 into cavity 10, in accordance with an embodiment of theinvention. As shown feed 16 include a coaxial feed 37 with its centerconductor 38′_having an extension that is formed into a helix. Thisantenna can be designed for matching into free space over a relativelywide band of frequencies (such as that useful for the present invention)and can be made more or less directional by changing the number ofturns. The free space design is then adjusted for the presence of thecavity as described below with respect to FIG. 4C. The graph of FIG. 4Cshows experimental results for a helix of 7 turns, with a diameter equalto the free space wavelength and a turn pitch of less than 0.2wavelengths. However, the present inventors have found that curves ofthe type shown in FIG. 4C can be found, by experimentation, for otherturn characteristics as well.

Fractal antennas are known in the art. Reference is made to Xu Liang andMichael Yan Wan Chia, “Multiband Characteristics of Two FractalAntennas,” John Wiley, M W and Optical Tech. Letters, Vol. 23, No. 4, pp242-245, Nov. 20, 1999. Reference is also made to G. J. Walker and J. R.James, “Fractal Volume Antennas” Electronics Letters, Vol. 34, No. 16,pp 1536-1537, Aug. 6, 1998. These references are incorporated herein byreference.

FIG. 4D shows a simple bow-tie antenna 50 as known in the art, forradiation into free space. The Bandwidth of the bow-tie (in free space)is: 604 MHz @ 740 MHz center frequency (−3 dB points) and 1917 MHz @2.84 GHz center frequency. This antenna has a monopole directivitypattern but a broadband one (being an advantage over the narrow BW of adipole antenna). However, monopole directivity does not irradiate in adirection parallel to the feed.

The band width (BW) of this antenna varies between 10 MHz and maximum of70 MHz depends of the load (object) position inside the cavity.

This and the following fractal antennas can be useful in the presentinvention to feed energy into a cavity.

FIG. 4E shows a simple Sierpinski antenna 52, useful in the practice ofthe present invention. Generally, the cross-hatched areas 54 are metalplate and the white central area 56 is a non-conducting region. Themetal plates are mounted on a preferably low dielectric constantdielectric and are connected at the corners and to center conductor 38of coaxial feed 37, as shown. It's characteristics in the cavity aresimilar to those of the bow-tie antenna.

FIG. 4F shows a modified Sierpinski antenna 58, useful in the practiceof the present invention. Generally, the cross-hatched areas 60 aremetal plate and the white areas 62 are non-conducting regions. The metalplates are mounted on a preferably low dielectric constant dielectricand are connected at the corners and to center conductor 38 of coaxialfeed 37 as shown.

For an overall extent of 103.8 mm utilizing equal size equilateraltriangles, the center frequency of this antenna is about 600 MHz insidethe cavity.

FIG. 4G shows yet another modified Sierpinski antenna 64, useful in thepractice of the present invention. Generally, the cross-hatched areas 66are metal plate and the white areas 68 are non-conducting regions. Themetal plates are mounted on a preferably low dielectric constantdielectric and are connected at the corners and to center conductor 38of coaxial feed 37.

Dimensions are shown on FIG. 4G for an antenna having a center frequencyof 900 MHz in the cavity.

FIG. 4H shows a multi-layer fractal antenna 70 made up of three fractalantennas spaced a small distance (e.g. 2 mm) from each other.

The size of each of these antennas is staggered in order to broaden thebandwidth of the antenna. In the example shown a first antenna 72 isscaled to 0.8 of the dimensions given in FIG. 4G. A second antenna 74has the same dimensions as the antenna of FIG. 4G and a third antenna 76is increased in size over antenna 74 by a factor of 1.2. The volumefractal antenna (FIG. 4G) has an overall bandwidth of 100 MHz—this is animprovement over the 70 MHz maximum BW achieved in prior single fractalantenna (FIGS. 4D-4H).

Fractal antennas also show a center frequency change when placed in acavity. This difference is used (as with the helical antenna to designantennas for use in cavities by scaling the frequencies.

In general, it is desired to utilize wideband, directional antennas tofeed power into the object being heated such antennas include patchantennas, fractal antennas, helix antennas, log-periodic antennas andspiral antennas.

FIGS. 5A to 5D are schematic block diagrams of an electromagneticheating system, in accordance with an embodiment of the invention.

FIG. 5A shows a general block diagram of each of the power feeds 90 ofthe system, in an exemplary embodiment of the invention. The system iscontrolled by a computer 92 which via a control interface (also referherein as Controller or control circuit) 130 controls an RF system 96which provides power to the heated object 98.

FIG. 5B is a block diagram of the electronics of one of the RF feedsystems 96, in accordance with an exemplary embodiment of the invention.A VCO 102 receives a signal from a control circuit 130 (FIG. 5C) whichsets the frequency of the energy into the port. This energy is passedthrough an RF switch 104 and a voltage controlled attenuator (VCA) 106,both of which are controlled by control circuit 130. After passingthrough the VCA 106, the power and frequency of the signal have beenset. A load 108 is provided for dumping the signal generated by VCO 102when the signal from VCO 102 is not switched to the VCA.

The signal is then sent through the main line of an optional first dualdirectional coupler 110.

The output of the VCA is then amplified by a power amplifier 112 andafter passing though an isolator 114. A signal proportional to the powerreflected from amplifier 112 is also fed to the control circuit.

Coupler 110 feeds back a portion of the signal entering it (afterdetection or measurement of power) to control circuit 130. A signalproportional to the power reflected by amplifier 112 is also sent tocontroller 130. These signals enable supervision of VCO/VCA and theamplifier. In a production system, the directional coupler may not benecessary.

An RF switch 116 switches the power either to a power load 118 or to thefeed of resonator 98, via a second dual directional coupler 120. Dualdirectional coupler 120 samples the power both into and out of theresonator and sends power measurement signals to controller 130.

In an embodiment of the invention, RF amplifier 112 is a solid stateamplifier based on the LDMOS technology. Psat=300 W, Efficiency=about22%, Effective band—800-1000 MHz. Such amplifiers either have arelatively narrow bandwidth or a low efficiency (<25%) or both. Thislimits the optimal utility of the advances of the present invention.Recently, amplifiers have become available based on SiC (siliconcarbide) or GaN (gallium nitride) semiconductor technology. Transistorsutilizing such technologies are commercially available from companies,such as Eudyna, Nitronex and others. Amplifiers having a maximum poweroutput of 300-600 W (can be built from low power (50-100 Watt) modules)and a bandwidth of 600 MHz (at 700 MHz center frequency) or a bandwidthof 400 MHz (at 2.5 GHz center frequency are available, for example. Suchamplifiers have a much higher efficiency than prior art amplifiers(efficiency of 60% is available) and much higher tolerance to reflectedsignals, such that isolator 114 can often be omitted for theseamplifiers. A particular configuration utilizing this type of amplifieris described below in conjunction with FIGS. 12A-D.

Turning now to FIG. 5C controller 130 comprises computer 92 whichperforms computations and provides a logging function of the system aswell as acting as a user interface. It also controls the rest of theelements in performing the calibration and control method of the flowcharts of FIG. 7.

Computer 92 is coupled to the rest of the system through an interface134 which is designed to provide communication to, for example, anALTERA FPGA 140, which interfaces with and provides control signals tothe various elements of the RF system. The Altera receives inputs (asdescribed above with respect to FIGS. 5A-5C), via one or moremultiplexers 136 and an A/D converter 138. In addition, it sets thefrequency and power of each of the feeds (also described with respect toFIGS. 5A and 5B) via D/A converters 142 and the positions of the fieldadjusting element optionally utilizing the method described with aid ofthe following flow charts. In a production system, the computer may notbe necessary and the Altera or a similar controller may control andprocess all the necessary data. In some embodiments of the invention,the frequency is swept as described below.

FIG. 6 is a simplified flow chart 150 of the operation of a heatingsystem having the structure described above. FIG. 7 is a simplified flowchart of calibration 160 of the system. As will be evident, the methodoperation and calibration of the system is also usable with only minorchanges for operating systems with lesser or greater numbers of powerfeeds and/or a greater or less number of matching elements.

At 152 an object, for example a frozen organ or frozen or non-frozenfood object, is placed in cavity 10. A calibration or adjustment routineis then optionally performed to set the variable elements in the system.These can include power output of the amplifiers 112 in each of thepower feeds to the cavity at each frequency, chosen to be transmitted,the finite set of sub-bands of frequencies of each VCO 102, the methodof providing energy at the various frequencies (for example sweep orother frequency variation, or the provision of a pulsed signal embodyingthe desired frequency and power characteristics), positioning of thematching elements (e.g., 22, 24), position of the heated object and anyother variables that affect the various characteristics of the heatingprocess, for example—the uniformity and/or efficiency of power transferto the object. A memory contains the criteria 156 for calibrating thesystem. Exemplary criteria are described below. Calibration is carried160 out to determine the new heating variables. An exemplary calibrationroutine is outlined in the flow chart of FIG. 7, discussed below.

After the new variables are determined, the new variables are set 158and heating commences 170.

Periodically (for example a few times a second), the heating isinterrupted for a short time (perhaps only a few milliseconds or tens ofmilliseconds) and it is determined 154, optionally based on a methoddescribed below, whether heating should be terminated. If it should,then heating ends 153. If the criterion or criteria for ending heatingis not met, then a decision may be taken whether the heating variablesshould be changed 151. If the variables are to be changed (act 151—YES)the calibration (or re-adjustment) routine 160 is entered. If not (act151 NO), the heating 170 is resumed. It is noted that during themeasurement phase, the sweep is generally much broader than during theheating phase.

Calibration routine 160 for each individual channel will be described,with reference to the flow chart of FIG. 7.

In order to perform calibration, the power is optionally set at a levellow enough 162 so that no substantial heating takes place, but highenough so that the signals generated can be reliably detected.Alternatively, calibration can take place at full or medium power.Calibration at near operational power levels can reduce the dynamicrange of some components, such as the VCA, and reduce their cost.

Each of the inputs is then swept 164 between a minimum and a maximumfrequency for the channel. Optionally, the upper and lower frequenciesare 430 and 450 MHz. Other ranges, such as 860-900 MHz and 420-440 canalso be used. It is believed that substantially any range between300-1000 MHz or even up to 3 GHz is useful depending on the heating taskbeing performed. When the broadband, high efficiency amplifiersdescribed above are used, much larger bandwidth of several hundred MHzor more can be swept, within the range of the amplifiers. The sweep maybe over several non-contiguous bands, if more than one continuous bandsatisfies the criteria for use in heating.

The input reflection coefficients S₁₁, S₂₂, and S₃₃ and the transfercoefficients S₁₂=S₂₁, S₁₃=S₃₁, S₂₃=S₃₂ are measured during the sweep andnet power efficiency is determined as (for port I for example), as:

η₁=1−(Reflected power from port 1+coupled power to ports 2 and 3)/Inputpower.

The present inventor has found that under many operating regimes it isdesirable to maximize certain criteria.

In a first embodiment of the invention, the maximum net power efficiencyfor each port is maximized, in the sense, that the net power efficiencyat a point of maximum efficiency within the sweep range is made as highas possible. The efficiency and the frequency at which the efficiency isa maximum is noted. Optionally, the width of the efficiency peak and aQ-factor are noted as well.

A second embodiment of the invention is based on a similar criterion.For this embodiment the area under each resonance peak of the netefficiency of transfer is determined. This area should be a maximum. Theefficiency, the center frequency of the resonance having the maximumarea and its width are noted.

In an embodiment of the invention, the criteria for determining if thevariables are properly set is when the peak net efficiency (firstembodiment) or the area or a width (second embodiment) is above somepredetermined level or a Q-factor is below some predetermined level. Forexample, there may be a restriction that the area above 60% netefficiency is maximized for each of the feeds.

It is noted that energy that is neither reflected nor transmitted to theother ports is absorbed either in the walls of the cavity or in theobject being heated. Since absorption in the conducting walls is muchlower than that in the object by a large factor, the net efficiency isapproximated by the proportion of the input power that is absorbed inthe object. It is also noted that the frequency of maximum netefficiency is not necessarily the same as the frequency at which thematch is best.

A search is performed for a position of the matching elements at whichthe net power efficiency at all of the feeds meets the criteria. This isindicated at boxes 156, 166, 168 and 172, which represent a searchcarried out by changing the positions and/or orientations of thematching elements. Standard search techniques can be used or a neuralnetwork or other learning system can be used, especially if the sametype of object is heated repeatedly, as is common for industrial uses.

When the criteria are met 168—YES, then the power is raised to a levelsuitable for heating. The power into the respective amplifiers isoptionally normalized to provide a same net power into the cavity (andtherefore, into the object) for each port at Box 174. Optionally, theleast efficient port determines the power to the object.

In an embodiment of the invention, the frequency is swept, optionallywhile adjusting the power. The term swept should be understood toinclude serial transmission of individual non-contiguous frequencies,and transmission of synthesized pulses having the desiredfrequency/power spectral content.

The present inventors have discovered that each frequency has maximalabsorption at a specific location within an object within a cavity,which locations may vary between different frequencies. Thereforesweeping a range of frequencies may cause movement of the peak heatingregion within the object, computer simulations have shown that, at leastwhen the Q factor of a peak is low (i.e., a lot of energy is dissipatedin the object being heated) the movement of the peak heating region canbe quite substantial. Furthermore, the inventors have found that eachmode (represented by a different peak of efficiency) acts differentlywhen swept.

FIG. 11A is a simplified flow chart 200 of a method of determining sweptpower characteristics, in accordance with an embodiment of theinvention. This method corresponds to acts 160 and 158 of the flow chartof FIG. 6.

After placing the object in the cavity (152) the cavity is swept todetermine the input efficiency as a function of frequency (202) (e.g.,obtain a spectral image). Determination of input efficiency is describedin detail above. Alternatively, a pulse of energy, having a broadspectrum in the range of interest is fed into the input. The reflectedenergy and the energy transmitted to other inputs are determined andtheir spectrums are analyzed, for example using Fourier analysis. Usingeither method, the net power efficiency as a function of frequency canbe determined.

Under some conditions, where similar objects have been heatedpreviously, a set of tables for different types and sized of objects canbe developed and used as a short-cut instead of closely spacedmeasurements.

FIG. 11B shows a simplified net power efficiency curve 1250 at an input.It is noted that there are regions in which the efficiency is high andothers in which the efficiency is low. Furthermore, some of theefficiency peaks are broader and others are narrower.

Next, the overall swept bandwidth (BW) is determined (204). This mayinclude sweeping across a single peak or across several peaks.

In an embodiment of the invention, during the heating phase, thefrequency is swept across a portion of each of the high efficiencypeaks. For example, to provide even heating of objects it is believedthat the power inputted to the cavity at each frequency should be thesame. Thus, in an embodiment of the invention, the power at eachfrequency is adjusted such that P*η is a constant for all thefrequencies in the sweep. Since the power available is always limited tosome value, this may set a limit on the available bandwidth for thesweep. An example of a lower limit to efficiency is shown as dashed line1252 in FIG. 11B. The sweep may be limited to frequencies havingefficiency above this value.

Next, the positions of the field adjusting elements are set. Thisadjustment is optional and in some situations, even where such elementsare present, they do not need to be adjusted. In general, the criterionfor such adjustment is that the peaks have as high efficiency aspossible with as broad a peak as possible Specific applications mayintroduce additional goals, such as moving the peak to a certain band.

An iterative process (206, 208) is used to determine a desired positionand/or orientation of the field adjusting elements. When the searchprocess which may be any iteration process as known in the art, iscompleted the elements are set to the best position found. (210).

In an embodiment of the invention, the sweep is adjusted (212) to avoidfeeding excess power into certain parts of the object. For example, ifthe object contains a metal rod or a metal zipper, a high peak inefficiency 1254 may be generated. A metal rod can cause a concentrationof energy near the ends of the rod. Avoiding irradiation at this peakcan sometimes reduce the effects of such objects on even heating.

Next, the sweep parameters are determined (214).

FIG. 11D shows the power spectrum 1256 of energy to be fed to the input,in accordance with an embodiment of the invention. It should be notedthat no energy is transmitted at the frequency characteristic of the rodand that for other frequencies for which the efficiency is above theminimum shown at 1252 in FIG. 11B, as illustrated in FIG. 11C. The powerhas a shape that is such that the product of the efficiency η and thepower fed is substantially constant.

In an alternative embodiment of the invention, the energy is fed to theport in the form of a pulse rather than as swept energy. First a pulse,such as that shown in FIG. 11D is generated by a pulse synthesizer. Thispulse is amplified and fed into the input. The pulse synthesizer wouldthen replace VCO 102 (FIG. 5B). It is understood that the pulsesynthesizer can also be programmed to produce a sweep for use indetermining the frequency dependence of η (act 164 of FIG. 7).

When the criteria are met, then the power is raised to a level suitablefor heating and optionally swept. The power into the respectiveamplifiers is optionally normalized to provide a same net power into thecavity (and therefore, into the object) for each port. Optionally, theleast efficient port determines the power to the object. While in priorart ovens, the user decides on the heating time, in some embodiments ofthe present invention the desired heating time can generally bepredicted.

Returning again to FIG. 6, there are a number of methodologies forperforming the heating 170.

In one embodiment of the invention, power is fed to all of the feeds atthe same time. This has the advantage that heating is faster. It has thedisadvantage that three separate sets of circuitry are needed.

In a second embodiment of the invention, the power is fed to the feedsseriatim, for short periods. Potentially, only a single set of most ofthe circuitry is needed, with a switch being used to transfer the powerfrom feed to feed. However, for calibration, a method of measuring thepower transmitted from port to port should be provided. This circuitrycould also be used to match the feeds when power is not being fed tothem. A different type of circuitry for providing both the heating andcalibration functionality, in accordance with an embodiment of theinvention, is shown in FIG. 8, corresponding to the circuitry of FIG.5B.

The same reference numbers are used in FIG. 8 as for FIG. 5B, except asindicated below. Such a system has the advantage of being much lessexpensive. It is, of course, slower. However, it does allow for anadditional method of equalization, in which the time duration (eitheralone or in conjunction with changing the input power) during which eachfeed is fed is adjusted so that the energy into each feed is the same(or different if that is desired).

FIG. 8 is similar to FIG. 5B up to the output of RF switch 116.Following RF switch 116 a second RF switch 192 transfers the powerdelivered by amplifier to one of the feeds. Only circuitry 220 relatedto feed 2 is shown.

Circuitry 220 operates in one of two modes. In a power transfer mode, asignal from control 130 switches power from RF switch 192 to dualdirectional coupler 120, via an RF switch 194. The rest of the operationof the port is as described above. In a passive mode, the input to RFswitch 194 does not receive power from amplifier 112. Switch 194connects a load 190 to the input of dual directional coupler 120. In thepassive mode, load 190 absorbs power that is fed from the cavity intothe feed. For production systems additional simplification ofdirectional coupler 120 may be possible, replacing the dual directionalcoupler with a single directional coupler.

It should be noted that switches 116 and 192 and optionally the localswitches can be combined into a more complex switch network.Alternatively or additionally, RF switch 194 can be replaced bycirculator such that power returned from the feed is always dumped inload 190.

In either the embodiment of FIG. 5B or the embodiment of FIG. 8, thefrequency of the power fed to a port can be fed at the center frequencyof the resonance mode that couples the highest net power, i.e., thepoint of maximum efficiency of energy transfer to the object beingheated. Alternatively, the frequency can be swept across the width ofthe resonance or, more preferably along a portion of the width, forexample between the −3 dB points of the power efficiency curve, or asdescribed above with respect to FIGS. 11A-11C. As indicated above,optionally, the power is adjusted during this sweep so that the netinput power remains constant or more nearly constant during the sweep.This can be accomplished by changing the power amplification of thepower amplifier inversely to the power efficiency of the instantaneousfrequency being fed.

Returning again to FIG. 6, reference is additionally made to FIG. 9,which shows a graph of frequency of a particular peak with time for atypical thawing process. This graph illustrates one method of using thechanges in the properties of the object during a thawing process todetermine when the process is complete.

The ordinate of FIG. 9 is the frequency chosen as an input for one ofthe feeds. The abscissa is time. During thawing of an object, the ice inthe object turns to water. Ice and water have different absorption formicrowave or UHF energy, resulting in a different return loss andcoupling as a function of frequency. Not only does this change thematch, but at least after rematching by adjustment of the matchingelements, the frequency of the absorption efficiency peak changes. Atpoint A, some of the ice has started to change into water and thefrequency of match changes. At point B, all of the ice has changed towater and the frequency of match stops changing. By monitoring thefrequency described above and especially its rate of change, the pointat which all of the ice is turned into water can be determined and theheating terminated, if only thawing is desired. It is noted that thefrequency change during thawing is large, as described herein, comparedto allowed frequency changes in the prior art.

One of the problems of thawing a solid mass of irregular shape andirregular internal structure is that it is generally impossible todetermine when all of the ice has been turned to water. Thus, ingeneral, in the prior art, one overheats to assure that no ice is left,which, considering the uneven heating of the prior art, would enhancere-crystallization, if any were left.

Heating methods and apparatus of the present invention, which allow forboth even heating and provide knowledge of the progress of the thawing,can result in much lower or even non-existent re-crystallization.

Apparatus and method according to the present invention have been usedto defrost a pig's liver, Sushi or Maki and to cook an egg in the shell.

The following table shows a comparison of thawing of a cow liver by thesystem of the present invention and using a conventional microwave oven.

TABLE 1 Comparison of Inventive Method and Conventional Microwave-CowLiver Inventive Measurement Method Conventional Microwave InitialTemperature −50° C. −50° C. Final Temperature 8° C. to 10° C. −2° C. to80° C. after thawing Power 400 Watt 800 Watt Thawing time 2 Minutes 4Minutes Visible damage None The texture of the thawed sample wasdestroyed. There are frozen regions along side burned ones. No chance ofsurvival of living cells.

The following table shows a comparison between thawing of Makicontaining raw fish covered by rice and wrapped in seaweed, by thesystem of the present invention and using a conventional microwave oven.

TABLE 2 Comparison of Inventive Method and Conventional Microwave-MakiInventive Conventional Microwave Method Measurement −80° C. −80° C.Initial Temperature −5° C. to 60° C. 2° C. to 6° C. Final Temperatureafter thawing 800 Watt 400 Watt Power 1 Minute 40 Seconds Thawing timeThe thawing process cooked None Visible damage part of the salmon,therefore it was not Maki anymore.

An egg was cooked using the present method. Generally, eggs burst if anattempt is made to cook them in a microwave oven. However, using thesystem described above an egg in the shell was cooked. The white andyellow were both well cooked, and the white was not harder than theyellow. Neither part was dried out or rubbery and the taste was verygood, with little if any difference from a conventional hard cooked egg.In addition, deep frozen fish have been defrosted without leaving anyfrozen portions and without any portions being heated above cookingtemperatures.

In each of the above experiments, the frequency and power were adjustedautomatically and the matching elements were adjusted manually, inaccordance with the method given above for automatic adjustment.

The inventors believe that the methodology of the present invention iscapable of thawing objects that are deep frozen to just above freezingwith a temperature variation of less than 40° C., optionally less than10° C., 5° C. and even as low a difference as 2° C. Such results havebeen achieved in experiments carried out by the inventors, for a cowliver, for example.

Thawing objects such as meat and fish with such low differences and athigh speed has the potential for prevention of development ofsalmonella, botulism and other food poisons. Controlled, uniform thawinghas important implications in thawing organs for transplanting, withouttissue destruction.

FIG. 10 shows apparatus for applying a DC or relatively low frequency(up to 100 kHz or 100 MHz) to an object in the cavity, in accordancewith an embodiment of the invention. This figure is similar to FIG. 1,except that the cavity includes two plates 250 and 252. A power supply(not shown) electrifies the plates with a high differential voltage atDC or relatively low frequency. The objective of this low frequencyfield is to reduce the rotation of the water molecules. Ice is water ina solid state therefore its rotational modes are restricted. A goal isto restrict the rotational modes of the liquid water in order to makethe heating rate be determined by that of the ice. The present inventorsalso believe that the low frequency fields may change the dielectricconstant of the materials making up the object being heated, allowingfor better match of the input to the object.

In an alternative embodiment of the invention a DC or low frequencymagnetic field is applied by placing one or more coils inside orpreferably outside the cavity to cause alignment of the molecules in theobject. It is possible to combine low frequency or DC electric and lowfrequency or DC magnetic fields with possible different phases fromdifferent directions.

FIG. 12A shows a cavity 198 with an internal heater coil 600 placedinside the cavity. An inlet 602 and an outlet 604 allow for feeding ahot fluid through the coil to heat the air within the cavity.

FIGS. 12B and 12C show two schematic illustrations of a system fortransferring heat from a high power amplifier 606 to the coil. Even atan efficiency of 60%, the amplifier can generate several hundred watts.This energy (or at least a part of it) can be transferred to heat theair and to produce infrared radiation (as a resistive coil does) in thecavity to increase the efficiency of heating.

FIG. 12B shows a very schematic diagram to illustrate how waste heatfrom an amplifier 606 can be captured. FIG. 12C shows a block diagram ofthe same system. Element 608 represents a cooling system for returningfluid and a fluid pumping system. It receives return fluid from outlet604, cools the liquid (if necessary) and pumps the liquid into a gap 610between the between amplifier 606 and an optional heat sink 612. Thetemperature at the input to the gap and at its output are preferablymeasured by sensors 614 and 616 and fed to a control system 618, whichcontrols one and optionally more than one of the cooling and pumpingrate to provide a desired heat transfer to the cavity. A fan may beprovided to cool the heat sink as necessary. The fluid passing betweenthe amplifier and the heat sink also functions to transfer heat from theamplifier and the heat sink. Optionally heat conducting rigs maytransfer heat between the amplifier and the heat sink with the fluidpassing between the ribs to collect heat.

Alternatively, heat pipes or other means can be used to collect andtransfer energy to the cavity. Alternatively, hot air could be passedover the amplifier and/or heat sink and passed into the cavity.

Use of high efficiency amplifiers with or without heat transfer to thecavity can result in highly efficient systems, with an overallefficiency of 40-50% or more. Since amplifiers with relatively high(40V-75V) voltages are used, the need for large transformers is obviatedand heat sinks can be small or even no-existent, with the amplifiertransferring heat to the housing of the heater.

By optimizing the system, a heater as shown in FIG. 12D, including ahousing 650, amplifiers and controller, as well as a user interface 652and a door 654, as normally found on a microwave oven can weigh aslittle as 10 or 15 Kg or less.

While applicants have utilized UHF frequencies for heating in theexamples described above, rather than the much higher 2.45 GHz used inthe prior art, for heating applications other than thawing, a differentfrequency may be desirable. UHF frequencies are absorbed preferentiallyby ice and have a longer wavelength than the higher frequencies, so thatthe fields within the object are more uniform and the ice ispreferentially heated as compared to the water. This provides forpreferential heating of the ice and more even thawing.

Additional measures that may be taken to improve the uniformity are:

1) Various types and sizes of conducting materials such as tiny grainsof powdered conductive material (gold) may be inserted into the samplepreceding the freezing process (e.g. through the circulation of theblood or cooling fluid) and serve as reflecting sources. The insertioncan be done using some template of non-conducting material (absorbing ornot) holding the conducting objects. These passive energy sources canimprove the uniformity of EM radiation absorption.

2) Penetration of materials that change their dielectric characteristicsdependent upon temperature in a fashion that is different than that ofthe sample. Injecting these materials will enable changes in thedielectric characteristics of the sample in the direction desired forachieving uniform and fast warming.

3) Use of probes for measurement of various parameters of the warmingprocess such as temperature, pressure, and so on: These probes can beinserted inside the sample preceding the freezing process or attachedadjacent to the sample at any stage of the process. Measurement of theseparameters provides a means for supervision (control) of the warmingprocess such that if the warming is not optimal it will be possible tomake changes in various parameters of the process. There are probesavailable that are suited for measurement during warming in a microwavedevice. These probes can also serve as an indication of when to stop athawing or cooking process.

Such probes may be included in a bag in which the object to be heated isplaced and may include a resonant element whose resonant frequency ismade to vary with temperature by the inclusion of a temperaturedependent element such as a temperature dependent resistor or capacitor.

Probes may be provided with resonant circuits whose frequency depends ontemperature. Such probes may be scanned during the scanning used forsetting sweep parameters to determine temperature. During powertransfer, these frequencies should generally be avoided. In anembodiment of the invention, a temperature sensitive tag is paired witha temperature insensitive tag and the changes in the frequency of thetemperature sensitive tag are determined by a difference frequencybetween the two. This allows for a more accurate measurement oftemperature that utilizing an absolute measurement of the frequency ofthe temperature sensitive tag.

4) Wrapping of the sample in material that does not absorb EM radiationat the specified frequencies: This type of wrapping can serve aspackaging for the sample during transportation and as part of the probesystem by which it is possible to measure temperature and additionalparameters at the edges of the sample. This wrapping can serve as localrefrigeration for the outer surfaces of the sample (which usually have atendency to warm faster than the rest of the sample) in order to achieveuniformity in the warming of the sample.

Further, the wrapping can include identification of the object to helptrack the object and also to provide an indication to the system of apreferred protocol for heating the object. For example the wrapping maybe provided with a number of resonant elements which can be detectedwhen the cavity is swept during calibration. The frequencies of theelements can be used to provide an indication of the identity of theobject. This allows for the automatic or semi-automatic setting of thestarting parameters for calibration and/or for a particular heatingprotocol, optimized for the particular object and conditions.

Alternatively or additionally, to resonant circuits, a recording/storageelement of a different type is provided, for example, in the form of anRFID element or a bar-code, which includes thereon an indication of thecontent of a package or wrapper including the object, suggestedtreatment thereof and/or heating instructions. In an exemplaryembodiment of the invention, the instructions are actually provided at aremote site, indexed to a key stored by the recording element. Suchinstructions may be, for example, stored in a table or generatedaccording to a request, based on information associated with theidentification.

A reader is optionally provided in the heater, for example, an RFIDreader or a bar-code reader to read information off a package or awrapper thereof.

In an exemplary embodiment of the invention, after the object isprepared, various types of information are optionally stored on (or inassociation with) the recording element, for example, size, weight, typeof packing and/or cooking/thawing/heating instructions.

In an exemplary embodiment of the invention, the recording element hasstored therewith specific cooking instructions. Alternatively oradditionally, the recording element has stored therein informationregarding the platter shape and/or dielectric properties of itscontents. It is noted that for industrial shaped portions, if the shapeof the food is relatively regular between platters, movement of the foodand/or changes in size and/or small changes in shape will not generallyaffect the uniformity by too much, for example, shifting a heatingregion/boundary by 1-2 cm. Optionally, the platter includes a depressionand/or other geometrical structures which urge the food item to maintaina desired position relative to the platter borders.

During heating of the food, the parameters of the heating are optionallyvaried. The effect of the varying may cause non-uniformity in spaceand/or in time. In an exemplary embodiment of the invention, a script isprovided which defines how and what to vary. Optionally, the scriptincludes decisions made according to time (e.g., estimation of aneffect) and/or food state (e.g., measurement). Various measuring methodsare described above. Estimation is optionally based on a simulation oron empirical results from previous heating cycles. Optionally, thescript is conditional (e.g., modified, generated and/or selected), forexample, based on the position of a platter in the oven and/or personalpreferences (which may be stored by the oven).

In an exemplary embodiment of the invention, a script is provided on therecording element or at a remote location. Optionally, a script isselected by a user selecting a desired heating effect.

In one example, a single food item may experience different power levelsfor different times, in order to achieve a desired texture/flavor.

In an exemplary embodiment of the invention, a script is used to setdifferent energy levels and/or different times to apply such energies.

In one example, a script is as follows:

(a) Heat all platter so that the food reaches a relatively uniformtemperature of 5 degrees Celsius.

(b) Uniformly heat whole platter at 80% for 5 minutes and then fullpower for 10 minutes.

(c) Heat to 40 degrees Celsius.

(d) Maintain heat for 10 minutes. It is noted that a desired heat canoptionally be maintained by estimating the energy absorption whileapplying a known amount of cooling. Alternatively, actual heatabsorption may be estimated based on a known amount of energy absorptionand a measurement of air temperature leaving the cavity. Optionally, theoven includes a source of cooling air and/or has coolable walls and/ortray.

(e) Reduce heat to 30 degrees Celsius.

(f) Wait 10 minutes.

(g) Report “done” but leave at 30 degrees Celsius until removed.

In an exemplary embodiment of the invention, the script includes otherconditions, for example, detecting changes in color (e.g., browning),steaming (e.g., by phase change of water), volume (e.g., dough risingwill change the behavior of the cavity in ways that can be anticipated).

Optionally, the script includes a request to the user to add ingredients(e.g., spices), or to mix or reposition object.

In an exemplary embodiment of the invention, the script takes intoaccount the quality of uniformity control achievable by the oven. Forexample, if a higher level of uniformity is desired than basicallyprovided by the oven, heating may include pauses where power is reduced,to allow heat to even out in the object. The length of the delays isoptionally pre-calculated for the food substances and a calibrated lackof uniformity of the oven. Alternatively or additionally to reducingpower, the food and/or the heating areas may be moved one relative tothe other so as to better distribute heating.

In an exemplary embodiment of the invention, no script is provided.Instead, the heating times and/or parameters are based directly on thedesired results, measured food properties and/or measured heatingproperties. Such desired results may be user provided or indicated bythe recordable element.

5) Liquid injection: (similar to cooling liquid) that is suitable for abiological sample, the purpose of which is to cause uniform warming:This liquid is used in the field of hyperthermia. In this field warmingof a biological area is done in order to remove a cancerous growth. Fromknowledge derived from this field it is possible to understand that aliquid such as this can cause a drastic change in the warming uniformityand can enable use of a warming device that is more simplified thanwould be required without its use.

6) Penetration of active radiation sources in the sample during thefreezing process: These sources are active, which means connected to anexternal supply line that will be used as a source of EM radiation thatwill emanate from within the sample.

The present invention has been described partly in the context ofthawing. The inventors believe that based on the results shown above, itcan be expected that the methods of the present invention, can be usedfor baking and cooking, areas in which conventional microwave ovens arenotoriously weak or for other heating operations, especially those forwhich a high level of uniformity or control is needed and/or in which aphase change takes place.

Utilizing various embodiments of the invention, the UHF or microwaveenergy may be deposited uniformly in an object to within less than ±10%,±20% or ±30% over 80% or 90% or more of the object.

The present invention has been described using detailed descriptions ofembodiments thereof that are provided by way of example and are notintended to limit the scope of the invention. The described embodimentscomprise different features, not all of which are required in allembodiments of the invention. Some embodiments of the present inventionutilize only some of the features or possible combinations of thefeatures. Variations of embodiments of the present invention that aredescribed and embodiments of the present invention comprising differentcombinations of features noted in the described embodiments will occurto persons of the art.

Furthermore, the terms “comprise,” include,” and “have” or theirconjugates shall mean: “including but not necessarily limited to.” Thescope of the invention is limited only by the following claims:

1. A method of delivering energy to an object in a cavity, the methodcomprising: transmitting electromagnetic energy to the cavity at aplurality of frequencies; at each of the plurality of frequencies,determining an efficiency of energy transfer into the object; andadjusting power transmitted at each of the plurality of frequencies suchthat a multiplicative product of the efficiency and the powertransmitted is substantially constant across each of the plurality offrequencies.
 2. The method of claim 1, wherein the plurality offrequencies are in a band, and wherein transmitting includes sweepingthe band.
 3. The method of claim 2, wherein a power level associatedwith one or more of the plurality of frequencies of the electromagneticenergy transmitted to the cavity is adjusted during the sweeping of theband.
 4. The method of claim 2, wherein the band is swept a plurality oftimes during transmitting.
 5. The method of claim 1, wherein adjustingincludes regulating energy transmission causing substantially evenheating of the object.
 6. The method of claim 1, wherein adjustingincludes calculating power absorbed by the object based on a known powerinput and a known efficiency of energy transfer to the object.
 7. Themethod of claim 1, further comprising altering at least onecharacteristic of a housing associated with the cavity in order toaffect the efficiency of power transfer.
 8. The method of claim 1,wherein adjusting of the power transmitted causes a predetermined netefficiency of energy transfer to the object.
 9. The method of claim 1,wherein adjusting of the power transmitted is carried out in order tothaw the object.
 10. The method of claim 1, wherein transmitting occursat least until the object is thawed.
 11. The method of claim 10, whereina temperature differential throughout the object, when thawing iscomplete, is less than 5° C.
 12. The method of claim 1, wherein saidadjusting includes varying the transmitted frequency over a sub-bandwithin a primary frequency band, wherein the sub-band has a widthgreater than 0.5% of a center frequency of the primary frequency band.13. The method of claim 1, wherein adjusting includes varying energy atthe plurality of frequencies over a sub-band within a primary frequencyband, wherein the sub-band has a width greater than 20% of a centerfrequency of the primary frequency band.
 14. The method of claim 1,wherein the plurality of frequencies cover a band width of at least 100MHz.